· web viewsouth pars gas field is located in persian gulf (figure 1). the geological layer of...

Petrotex Library Archive

American Journal of Oil and Chemical Technologies

Journal Website: http://www.petrotex.us/2013/02/17/317/

American Journal of Oil and Chemical Technologies: Volume 2. Issue 5. May 2014

Subsurface Fracture Evaluation Using Image Log and Its Comparison with Core Data in South Pars Gas FieldMohammad Shalafi1, Siyamak Moradi1, Mohammad Kamal Ghasemalaskari1, Malihe Sadat Kazemi2

1. Department of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran.2. Technical and professional university, Tehran, Iran.

Abstract:

A fracture is a naturally occurring macroscopic discontinuity in rock due to deformation. The structure of fracture has a large effect on many petrophysical properties. FMI is an image log that provides high resolution images of the wellbore that are based on the electrical resistivity of different layers of the Earth. Kangan and Dalan formations are the main reservoirs of South Pars gas field. Evaluation of conventional log and MULTIMIN interpretation indicate that K2 member in Kangan formation and K4 member in Dalan formation, are the reservoir members. In order to evaluation of structural features such as open fractures, close fractures and stylolites, processing and interpretation were done by using FMI log and then correlated to core data. Finally, 112 open fractures, 47 close fractures and 76 stylolites were observed over the interpreted interval.

Keyword: Image log, FMI, Fracture, South Pars gas field, Stylolite.

1. IntroductionDue to plate tectonics, lots of fissures and fractures are created in subsurface layers, some of these fractures are open apertures, and some are closed [1]. Carbonate reservoirs have low matrix permeability, but as a result they became fractured and they can be productive [2]. Most of the reservoirs in Middle East are carbonate rocks, although some less common lithology is sandstone [2]. Carbonate rocks are too complex, so their interpretation and their distribution are difficult. If their difficulties are recognized, their modeling of statics and dynamics could be done better [3]. According to their complexity and significance of carbonate reservoirs in Iran, their studies are challenging in detail [4]. Different methods are available for evaluation of fractures in reservoir rocks such as core analysis, well log evaluation, seismic and well test evaluation [5].Mostly, the Formation Micro Imager (FMI) is operated to interpret the sedimentary environment of a reservoir rock [6]. The majority of Iranian hydrocarbon reservoirs are carbonates, thus detection and evaluation of fractures are crucial. Core evaluation is the best method of fracture studying; however, it is both technically and financially hard to do in the whole thickness of the reservoir and in all wells [7]. Geological structure is a key parameter determining the maturity of fractures [8]. The Formation Micro Imager provide a detailed view of stratigraphy, lithology, and structural informations along the wall of the wellbore [9]. The FMI tool provides an image with 80% coverage of the borehole wall in an 8-inch diameter borehole. The FMI tool offers a maximum resolution of 0.2 inches in the azimuthal and vertical directions and a detection limit of 50 microns (about 0.002 inches) in the horizontal direction [10]. The main target of this study is to investigation of petrophysical parameters such as effective porosity, water saturation and lithology by MULTIMIN interpretation, and detection of fractures in Kanga and Dalan formations.

2. Gelogical SettingThe stydy was carried out on South Pars gas field which is the most giant known gas filed in the world. South Pars gas field is located in Persian Gulf (Figure 1). The geological layer of South Pars reservoirs are consist of Kangan and Dalan formations. Previously studies have divided the Kangan and Dalan formations into four members, which are K1, K2, K3 and K4. The main lithology of Kangan and Dalan formations consist of calcite and dolomite, like most formations in Middle East [11].

http://www.petrotex.us/2013/02/17/317/

Authors /American Journal of Oil and Chemical Technologies 5 (2014) 45-47

Figure 1. Geographical location of South Pars gas field in Persian Gulf [11].

3. FracturesFractures are playing important role in reservoir modeling. Due to plate tectonics, lots of fissures and fractures are created to subsurface layers, some of these fractures are open apertures, and some are closed. Carbonate reservoirs have low matrix permeability, but as a result they became fractured, they can be productive. Fractures in borehole typically categorized into two groups: open and closed fractures.3.1 breakoutWellbore breakouts are now widely recognized as reliable indicators of subsurface in-situ horizontal stresses, of their orientations and, to a lesser degree, magnitudes Borehole breakout occurs when the stresses around the borehole exceed that required to cause compressive failure of the borehole wall.3.2 Drilling induced fractureDrilling Induced Fractures, grow as the borehole pressure exceeds the fracture pressure. In an intact rock, the drilling induced fracture growth occurs in two phases: fracture initiation and fracture propagation (Morita et al., 1996). Fracture initiation occurs when the borehole pressure exceeds the fracture initiation pressure and results in formation of small drilling induced fractures near the borehole. Fracture propagation occurs when the borehole pressure is maintained above the fracture propagation pressure. During the fracture propagation phase, the formed deilling induced gracture grows significantly in size (van Oort and Vargo, 20

4. Data and MethodThere are two major methods to perform petrophysical analysis by using well log data: Deterministic (classical) method and MULTIMIN (probabilistic) method. By MULTIMIN method, using all available logs data for interpretation of unknowns such as minerals volume, fluid volume, is possible. MULTIMIN method is more accurate than deterministic method, based on statistics and probability equations [11]. The initial orientations analyses, such as neutron-density and M-N plots are generated for different zones. In this study, lithology, water saturation, gas saturation, effective porosity are derived from MULTIMIN interpretation.Image log data are not conventional log data sets, and commonly require special software for image processing and interpretation. Image logs depending on the service company and the tool type have specific number of pads. One of the pads in FMI tool has to be defined as reference pad or PAD1 azimuth (P1AZ).Data processing is essential to convert the raw acquisition data into a high quality image for precise interpretation of features. In this study, GEOLOG 7.1 is used for image processing and interpretation. Data load is the first module that used to load the raw data. Important FMI data include cable speed (CV), tool deviation (DEVI), hole azimuth (HAZI), pad one azimuth (PAZ), caliper data (C1 and C2), pads and flaps microresistivity data and GPIT data [6]. The major steps of image processing in GEOLOG software are shown in Figure2.

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Fig. 2 Image processing workflow in GEOLOG Software [12].

The quality control log (ACCQ) and magnetometer quality control log (MAGC) should be checked for quality control. The Magnetometer quality control should be used for vertical and near vertical wells to assess the quality of X-axis and Y-axis of magnetometer logs. If the tool has rotated while logging the X and Y, magnetometer logs should show a circular distribution, center around the origin (0, 0). Specific applications of FMI logs are fracture identification, analysis of small-scale sedimentation features, evaluation of net pay in thinly formations, and the identification of breakouts and tensile analysis.

Plotting a dip alongside the other dips with appropriate display allows trends, boundaries and patterns to be identified. Bed boundaries, beds, slump features, faults and fractures are easily seen in the image logs. To pick the dipping traces in the image logs, the logs were already enhanced by log processing. Dips on this case study image log are picked and classified manually. Different dipping features were identified and picked for sedimentary and structural interpretation. To compute the dip of any planar feature seen on an image, the following is needed:

1) The relative position of 3 points on the plane.2) Orientation of the tool knowing the azimuth of pad #1 (P1AZ) as the magnetometer measures the angle between pad #1 and the magnetic north.3) Angle and direction of deviation of the borehole/tool.

5. Results and Discussion

For primary evaluation of the South Pars field, neutron density crossplots were studied. The initial analyses, such as neutron-density and M-N cross plots are generated for different zones. The best method available for modern, simple, porosity log analysis involves the density neutron crossplot. M-N cross-plot is also used for lithology investigations. Cross-plot diagram is obtained from the combination of three neutron, density, and sonic logs. In this diagram, parameter M indicates the extent of Density-Sonic and parameter N is the slop of cross-plot Neutron-Density where parameters M and N obtained according to the following relations while plotted against each other The investigation are showing the existence of light hydrocarbon in formations. Neutron-density and M-N cross-plot after applying corrections for Dalan and Kangan formations are shown in Figure 3.

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The terms M and N are defined as follows:

(1)

(2)

.

Figure3. Neutron-density and M-N crossplots for Dalan and Kangan formations

The dominant lithology of K1 member is calcite, dolomite and anhydritic dolomite and K2 member is consist of calcite and dolomite. The main dominant lithology of K3 and K4 consist of calcite and dolomite with some anhydrite intervals. Depth intervals, effective porosity and water saturation of K1, K2, K3 and K4 members are presented in table 1.

MULTIMIN interpretations is used to interpretation of lithology and fluid saturation of the reservoirs. The outputs of MULTIMIN interpretation is shown in Figure 4. In the first track, bit size, caliper log and gamma ray log are plotted.Also neutron,density PEF and HRLA logs are plotted in second track and resistivity logs, ae plotted in third tracks.The Caliper Log is a tool for measuring the diameter and shape of a borehole. It uses a tool which has 2, 4, or more extendable arms. The arms can move in and out as the tool is withdrawn from the borehole, and the movement is converted into an electrical signal by a potentiometer. In this well, at the depth of 3041m to the depth of 3060m, hole diameter is more than the bit size due to formation weak and cave in.

The gamma ray log can be used to calculate the presence of shale volume in formation. In this well, the values of gamma ray is not high, thus shale amounts are negligible in K1, K2, K3 and K4 members. also density-neutron and M-N crossplots can be used to shale colum analysis.

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Figure4. MULTIMIN interpretation of K and Dalan formations

Calcite, dolomite and anhydrite are the dominant lithology of K1, K2, K3 and K4 members. The K2 and K4 members consist of calcite and dolomite with lesser amounts of anhydrite. The lithological variations from calcite to dolomite and anhydrite control the reservoir properties. From the depth of 2972m to the depth of 3005m in K2 member, the gas saturation is high. From the depth of 3142.5 to the depth of 3208m in K4 member, the lithology is calcite and the gas saturation is high. There is a good correlation between the lithology from MULTIMIN interpretation and core data. Table 1 demonstrates the important parameters of MULTIMIN interpretation.

Table 1 effective porosity and water saturation values for Kangan and Dalan formations

Interval Depth (m) Water saturation (%) Effective porosity (%)K1 2895-2966 58 4.5K2 2966-3010 30 8.2K3 3010-3133 67 2.6

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K4 3133-3280 39 39

FMI is an image log providing high resolution images from the wellbore which are obtained based on the electrical resistivity difference existing among different layers. This enables geologists and petrophysicists to distinguish small scale phenomena in the wellbore which can be used to help dramatically the field development studies.

One of the main applications of FMI logs is detection of different types of fractures and their relevant parameters such as fracture direction and aperture. Processing of the FMI fata was done using GEOLOG software.

The quality of the images is quite good for geological analysis and almost over the entire logged section. However, there are few intervals where bad hole conditions affected the image quality. Such intervals include 2857-2860.5 m, 3039.5-3042 m, 3044.5-3049.5 m, and 3239-3241 m. FMI log precisely discriminated induced and natural fractures. Stylolites, open fractures, closed fractures, tensile and breakouts were interpreted from image log. The examples of interpreted fractures from FMI data compared to core data at the scale of 1/8 are shown in Figures 5 and 6.

Figure5. Open fracture at the depth of 2899.9m (the dip of the fracture is about 46 degree)

Figure6. Partially closed fracture (by anhydrite) at the depth of 2933.1m

In general, shear fractures occur in the direction of minimum stress and tensile fractures occur in the direction of maximum stress. Fractures due to their planar structures are observed in FMI image logs as sine waves. Black sine waves represent the open fractures and bright sine waves indicate filled fractures. In addition, amplitude of the sinus shows the fractures inclination. Interpretation of structural features at the scale of 1/700 is for Dalan and Kangan formations are shown in Figure 7.

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Figure 7. Interpretation of structural features in Dalan and Kangan formations

In Figure 7, olive drab tadpole represents open fracture, green tadpole, represents closed fracture and pink tadpole represents stylolites in Dalan and Kangan formations. Also, the dip and azimuth of each fracture, are shown in third track. Total of 112 open fractures and 47 closed fractures and 76 stylolites have been identified through Kangan and Dalan formations. Seventy six stylolites are identified over the interpreted interval. These stylolites appear mostly as conductive irregular surfaces on the images. Core data also, confirms the image result and there is a good correlation between core data and image log. The integration of core data with image log can improve the validation of result.

6. Conclusion

The primary analyses, such as neutron-density and M-N plots are generated for different zones. The investigation are showing the existence of light hydrocarbon in K2 and K4. The shale amounts are negligible in these zones. MULTIMIN interpretation indicates that the main lithology of Kangan and Dalan formations are calcite and dolomites and the amount of effective porosity in reservoir zones are not very high. Anhydrite were visible in FMI

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log and core, but there were not determined in MULTIMIN interpretation, thus the high accuracy of FMI is considered in this investigation. Open fractures, closed fractures and stylolites were interpreted in South Pars gas field and finally, 112 open fractures, 47 close fractures and 76 stylolites were observed over the interpreted intervals. Also, the interpreted fractures are striking towards NW-SE.

7. Refrences

[1] J. Kemeny, “The time-dependent reduction of sliding cohesion due to rock bridges along discontinuities: a fracture mechanics approach”, Jornal of Rock Mechanics and Rock Engineering, 36:27-38,2003.

[2] J.W. Buza, “An overview of heavy and extra heavy oil carbonate reservoirs in the Middle East”, International Petroleum Technology Conference, 2008.

[3] Q.M. Sadeq, S.K. Bhattacharya, W. Ismail, “Conceptual Fracture Modelling For Carbonate Reservoir in Bai Hassan Oil Field Northern Iraq”, Journal of Aquaculture Research & Development, 2016.

[4] M.I Al-Marzouqi, S. Budebes, I. Bush, R. Griffiths, K.B. Gzara, R. Ramamoorthy, R. Husser, Z. Jeha, J. Roth, ” Resolving carbonate complexity”, 22:40-55, 2010.

[5] S.D. Gupta, R. Chatterjee, M Farooqui, “Formation evaluation of fractured basement, Cambay Basin, India”, Journal of Geophysics and Engineering, 9-162, 2012.

[6] A. Shahinpur, “Borehole image log analysis for sedimentary environment and clay volume interpretation”,M.Sc. dissertation, Norwegian University of Science and Technology, 2013.

[7] V. Zaree, M.A Riahi, F. Khoshbakht, “Estimating fracture intensity in hydrocarbon reservoir: an approach using DSI data analysis”, Jornal of Carbonate and Evaporiates, 31:101-107, 2016.

[8] L. Zeng, W. Lyu, J. Li, L. Zhu, J. Weng, F. Yue, K. Zu, “Natural fractures and their influence on shale gas enrichment in Sichuan Basin, China”, jornal of Natural Gas Science and Engineering, 30:1-9, 2016.

[9] G. Aghli, H. Fardin, R, Mohamadian, G. Saedi,” Structural and fracture analysis using EMI and FMI image Log in the carbonate Asmari reservoir (Oligo-Miocene), SW Iran”, Jornal of Geopesia, 4:169-184, 2014.

[10] M. Shafiezadeh, M. Ziaee, B. Tokhmchi, “A New Approach towards Precise Planar Feature Characterization Using Image Analysis of FMI Image: Case Study of Gachsaran Oil Field Well No. 245, Southwest of Iran” Jornal of Petroleum Science and Technology, 5:51-58, 2015.

[11] V. Tavakoli, H. Rahimpour-Bonab, B. Esrafili-Dizaji, “Diagenetic controlled reservoir quality of South Pars gas field, an integrated approach”, Jornal of Comptes Rendus Geoscience, 343:55-71, 2011.

[12] C. Stelting, W. Schweller, W. Corea, W. Crane, L. Goggin, “Method for creating a stratigraphic model using pseudocores created from borehole images” , Google Patents, 2008.

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· web viewsouth pars gas field is located in persian gulf (figure 1). the geological layer of...

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